Recovery of phthalic acid from gases containing phthalic anhydride



Patented 1940 7 UNITED];

V y anchor I q j nnoovnnrk on; ,rn'rnsudr non), rRoM GASES con'rsmmo. .PHTHALIG ANHY- DRIDIII 6 ohms; (c tin -525i p preferably ata temperature. between about 35? C.

This invention relates to improvements'inthe removal of phthalic anhydride f rornggases by contact of the gases withaqueous liquids.

The invention is particularlyjconcerned with the recovery of phthalic anhydride from a reactiongas, such as that obtained in the vapor phase oxidation of polynuclear aromatic hydrocarbons, wherein the gas issbrought into intimate contact with an aqueous phthalic acid solution to form r a phthalic acid slurry. Such processes are described inUnited States Patents 2,071,329 and The present invention hasiorits purpose an improvement in the nature of the slurries obtained, whereby they may be handled with greater facility in contact apparatus, pipe,lines,treatment vessels, pumps, etc., and whereby, any tendency toward clogging of such apparatus may-be avoided. g, r

, When phthalic-anhydride-containing gas a is brought into contact with a solution consisting of water saturated with phthalic acid, especially such a solution containing otherreaction prodnets of the vapor phase catalytic oxidation of polynuclear aromatic compounds, such as naphthalene, the phthalic. acid formedby contact, of the anhydride and water hasa tendency to agglomerate in the form of crusts and lumps; If theprocess is operated by bubbling the gas through a body of liquid, crusts may form, around or within the outlet, end of the dip pipe. Agglomeration on the other hand may result in clogging of the recovery system, or where a packed contact tower is employed, in clogging of the packing.

By the process of the present invention -phthal-,

ic anhydride is removed from a gas containing phthalic anhydride vapor by contacting said gas with an aqueous; liquid containing a wetting agent. l

By the use of an aqueous absorption liquid containing a ,wetting agent, I have found that the formation of incrustations and lumps of gases of varied phthalic anhydride content such as are formed in vapor phase oxidation processes of theaforementioned type and it is capable of producing slurries containing from only a few per cent up to per cent and more, of solid phthalic acid. The aqueous liquid is maintained and about 70 C(but temperatures above or below thisrangegmay be employed. Suitable heat exchange apparatuslmaybe provided to main- 1 tain thel' preierred'temperature in those cases where its maintenance isgnot a normal result of a balanced heatexchange between the gas, absorption liquid and surrounding atmosphere.

Any wetting agent which is compatiblelwith the phthalic acid maybe employed. -Wetting or dispersing agentsv are generally sulfuric acid or phosphoric Q acid derivatives either, of aliphatic compounds (including hydrocarbon chain compoundsand chain compounds in which the chain ,isinter'ruptedby another group such as an. ester group) tor of aromatic compounds which may or may not be substituted. .Such' material's'jmay be present in'the. aqueous solution. as the free-acids or} as their wateresoluble salts although it is probable that the phthalic acid present converts fat least a part of the. salt to the freeacid. A

jwetting agent should be selected which doesnot l causeioaming of the aqueous solution since such foaming results inpriming and interferes with the e'fl'icient recovery of the phthalic acid,

Examples ofsuitable wetting agentsarealkyl naphthalene suli'onic, acids, such as,the isopro- "pyl'a'ted' and" butylated naphthalene-sulfonic acids, sulfonic acids of formaldehyde-naphthalene or formaldehyde-phenol condensation products, sulfated higherfatty acids, such as Turkey v red oil, etc. The proportion of wetting agent present in the aqueous absorption liquid depends upon the effectiveness of the individual wetting agent employed. Thus, for wetting agents such as sulfonated castor oil, concentrations between about .3 and, about 0.5 part by weight of the wetting agent per 100 parts by weight of water may be used to advantage. 1 I

' The phthalic-anhydride-containing gas may be contacted with the aqueous liquid in any convenient contact apparatus, for example, appara- ,tus arranged for bubbling the gas through the liquid or spraying-the liquid into the gas or exposing streams or films of the liquid to the gas. The following example illustrates the process of the present invention as applied to the recoveryof phthalic anhydride fromuhot reaction gases'obtained by the vapor phase catalytic oxi-- anhydride, together with carbon dioxide, nitrovacuum-jacketed or otherwise insulated pipe ex- 'phthalic acid crystalsj'is obtained, This slurry lowing the first to catchiany. spray whiChLma'y be carried along with the gas'leaving theffirstjcon- -tact unit. Solution from the second unit maybe second contact unit it may be advantageous to that entrainment ofyany additional spray ,is avoided; r I

essentially free of phthalic or maleic acid or anhydride and may be exhausted to the atmosphere.

I claim:

1. In the removal of phthalic anhydride from ,a gas.-..,containing phthalicanhydride vapor by gen, water vapor, etc., after cooling from reaction temperature to about 200 C., is introduced by means of a dip pipe (which may be a hausting .6 or 8. ,inches beneath the surface-oi surrounding liquidYinto a body poi watencontaining about 0.5 part by weight of commercial sulfonated castor oil per 100 parts of water. temperature of the water rises to about40 C and remains fairly constant at about thisfftem perature as a result of the balance between heat added by incoming gas and heat withdrawn by evaporation of water and by radiation. ,Pl 1;thalic anhydride in the gas reacts immedia-t withithe water to form phthalic acid which,.1s -retainedr in the aqueous liquid. Any maleic anhydride present in the gas is simultaneously dissolve I the aqueous absorption liquid. In this way e concentration of phthalic acid is built: upiuntil'; a smooth slurry containing betweenabout 10 per cent and about 12 per .cent 'ofsuspended improvement which comprises maintaining in the aqueous liquid a wetting agent.

2. In therecovery of phthalic anhydride in the I or eeus phthalic acid slurry from a metgas-"conta ning phthalic anhydride vapor, the improvement which comprises passing the gas qintelcontactgwith anaqueous phthalic acid slurry containingwa wetting agent.

;3. .In there'covefy of phthalic anhydride as an queous phthalic acid slurry from a hot gas coninirigfphthalicanhydride vapor, the improvement which comprises contacting the hot gas Withfan'iaqueous phthalic acid slurry containing a Wetting agent and maintained at a temperature'betweeni about 35 C. and about C.

4. In the recovery oflphthalic anhydride from arhot reaction gas obtained by the catalytic vapor phase air oxidation of .a" polynuclear aromatic compound, the improvement which. comprises cooling thehotjreaction gas to about 200 Cf'and introducing; it. into contact with an aqueous phthalic acid slurry containing a non-foaming wetting agent-and, maintained at a temperature "between about,35f" C. .and ab o1it '70. C. 1

may be drawn off, filtered to removeisolid'p'hthalic acid, and the filtrate returned to thecont'act apparatus, continuously or intermittently. A portion of the filtrate may befdrawn'offfeither continuously or intermittently for recovery of maleic acid and/or recovery or dispo'sal of other Icy-products. I, I e

A second contact unit containing a siinila'r vconcentration of wetting agent'may' be provided folgas containing phthalic anhydride vapor by contact of the gaswithan aqueous liquid, the improvement.whicltcomprises employing as the 'aqueou's'liquid a phthalic acid solution containing an aliph'aticfsul fon'ate etting agent of the "nonfoaming type. i v

6. In the recovery'of phthalic anhydride from passed periodically or continuously into'the first unit. By maintaining a low phthalioand 'maleic acid concentration'in the second 'unitjan'y spray carried to the atmosphere by the exhaust gases will be essentially free of phthalic and maleic e x acids. Instead of employing a dip'pipe in the phase air oxidation of naphthalene, the improvement which comprises cooling the hot reaction gas to aboutt200 C. and introducing it into contact with an'aque'ous phthalic acid slurry containing ab0ut" of Isulfonated castor oil and maintained'rt a temperature between about 35 C. and 'about 70'C.f

employ a contact apparatus wherein the g as merely passes over the surface oi the liquid so' The gas after treatment in "the second contact wa s. a a e p a re of about. 49' c; and'is GUY w. TALBERT.

,Qcj'ontact of ;the gas with an aqueous liquid, the

a hot reaction gas obtained by the catalytic vapor 5. In the removal of phthalic anhydride from 'a 

